There are two principal groups of the plasma sources: capacitively coupled plasma sources, which utilize RF electric field coupling to the plasma, and inductively coupled plasma (ICP) sources, which utilize RF magnetic field coupling to the plasma. Inductively coupled plasma (ICP) sources are commonly recognized as the most convenient and cost effective for plasma generation for plasma processing in semiconductor technology. The operation of the ICP is based on the principle of electromagnetic induction, in which RF current driven in a coil induces an electromagnetic RF field inside a vacuum chamber, ignites a plasma and sustains the plasma by induced RF plasma current. Currently the most common inductively coupled sources (ICP) have coils with a planar, cylindrical or dome-shaped geometries consisting of several loops to provide sufficient RF power into plasma.
Recently, antennae with more complex shapes have been proposed, for example, combined (hybrid) or dual coil configurations, coils generating torroidal plasmas, embedded coils, planar helicon (serpentine) antennae, 3D antennae, or parallel conductor antennae. A very common problem in ICPs for large area processing is relatively large radial plasma non-uniformity due to plasma diffusion to the walls of the chamber. Also, a significant problem is azimuthal plasma non-uniformity caused by the transmission line effects along the coil conductor, which are due to current non-uniformity along the coil length.
To improve plasma uniformity, more complex structures consisting of the multiple spirals connected in parallel and powered from common RF power sources have been proposed. However, tendencies to develop plasma instabilities have been observed in such configurations, thus generating even more inhomogeneity in a plasma.
Alternative approaches for ICP utilizing ferrite core transformers having a primary winding connected to an RF power source and a secondary winding being provided by a current conducting plasma have been proposed. But mechanical supports and arrangements, cooling of the individual inductors, and materials compatibility with process chemistry have made these concepts too complicated and impractical for high-density plasma applications, where proper and final solutions would be too costly. These facts have encouraged a search for alternative and simple solutions.
Subsequently, a planar low inductance coil has been proposed, in which an RF induction coil having a large surface area and a low profile has resulted in effective RF power coupling to the large load. Preferably, the turns of the coil comprise sheets of metal and the coil turns are substantially parallel to each other. However, it appears that the most effective power deposition into the plasma with such a coil occurs at a radius rather close to the inside of the coil, so the coil may not provide large size uniform plasma, but rather a center-peaked plasma density distribution. Consequently, a similar power density distribution can be achieved with significantly smaller spiral coil.
Accordingly, there is a need for an ICP source that produces a high density uniform plasma that is simple and low in cost.